As a system for assisting vehicle steering operation, an electric power steering system, which operates electrically to generate torque, is provided. An electric power steering system disclosed in, for example, JP 2008-44451A has an electric motor for applying steering assist force to a steering system, a motor drive circuit for driving a motor, a power supply relay circuit for supplying electric power from a battery to the motor drive circuit, and the like. The electric power steering system determines abnormality in a power supply path by monitoring a voltage level appearing at a side, which is connected to a power stabilizing capacitor in the power supply relay circuit.
A wire-break failure in the power supply path includes two cases, in one of which the power supply relay cannot be turned on and in the other of which a wire between a power source and the power supply relay is broken. According to JP 2008-44451A, it is checked whether the relay has an open-failure or a battery power supply wire has a wire-break based on whether the voltage level at the side connected to the power stabilizing capacitor of the power supply relay circuit rises. It is therefore necessary to monitor rise of the voltage level for a period of time to check whether the battery power supply wire has the wire-break or the power supply relay has the open-failure.